Deposition of amyloid-β peptide (Aβ) in brain occurs during normal aging and is accelerated in individuals with Alzheimer's disease (AD). Aβ is central to pathology of AD, and is the main constituent of brain parenchymal and vascular amyloid (1-6). Aβ extracted from senile plaques contains mainly Aβ1-40 and Aβ1-42 (7), while vascular amyloid is predominantly Aβ1-39 and Aβ1-40 (8). Several sequences of Aβ were found in both lesions (9-11). A major soluble form of Aβ, which is present in the blood, cerebrospinal fluid (CSF) (12-14) and brain (15-16) is Aβ1-40. In the circulation, CSF and brain interstitial fluid (ISF), soluble Aβ may exist as a free peptide and/or associated with different transport binding proteins such as apolipoprotein J (apoJ) (17-18), apolipoprotein E (apoE) (19), transthyretin (20), lipoproteins (21), albumin (22), and alpha-2 macroglobulin (α2M) (23).
The neuronal theory argues that soluble brain-derived Aβ is a precursor of Aβ deposits. Neuronal cells secrete Aβ in culture (24), which supports this view. An increase in soluble Aβ in AD and Down's Syndrome brains precedes amyloid plaque formation (15, 25, 26), and correlates with the development of vascular pathology (27). Several cytosolic proteases that may degrade intracellular Aβ in vitro cannot degrade extracellular Aβ from brain ISF (28) or CSF (29) in vivo. An exception is enkephalinase that may degrade Aβ1-42 from brain ISF (28). However, the physiological importance of this degradation in vivo remains still unclear since the peptide was studied at extremely high pharmacological concentrations (30).
It has been suggested that decreased clearance of Aβ from brain and CSF is the main cause of Aβ accumulation in sporadic AD (31). Since Aβ is continuously produced in the brain, a working hypothesis in this study was that efficient clearance mechanism(s) must exist at the blood-brain-barrier (BBB) to prevent its accumulation and subsequent aggregation in the brain. Cell surface receptors such as the receptor for advanced glycation end products (RAGE) (32-33), scavenger type A receptor (SR-A) (34), low-density lipoprotein receptor-related protein-1 (LRP-1) (35-38) and LRP-2 (39) bind Aβ at low nanomolar concentrations as free peptide (e.g., RAGE, SR-A), and/or in complex with α2M, apoE or apoJ (e.g., LRP-1, LRP-2). RAGE and SR-A regulate brain endothelial endocytosis and transcytosis of Aβ initiated at the luminal side of the BBB (33), while LRP-2 mediates BBB transport of plasma Aβ complexed to apoJ (39). The role of vascular receptors and BBB transport in the removal of brain-derived Aβ is unknown.
In the present study, a technique to measure brain tissue clearance in mice was developed based on a previous model in the rabbit (40). This technique was used to determine in vivo the efflux rates of Aβ1-40 from the CNS as a function of time and concentration of peptide, and to characterize vascular transport and/or receptor-mediated efflux mechanism(s) involved in elimination of brain-derived Aβ across the BBB. The study focused on LRP-1 and its ligands, α2M and apoE because both promote Aβ clearance in smooth muscle cells (35), neurons (36, 38), and fibroblasts (37); and the apoE4 genetic locus is definitely, and the α2M genetic locus is possibly associated with increasing the risk for AD (41-42).
This study can be used to improve the understanding of the pathogenesis of Alzheimer's disease and mechanisms of disease. New and nonobvious modes of diagnosis and treatment are suggested by this discovery. Other advantages of the invention are discussed below or would be apparent to persons in the art from the disclosure herein.